staging: comedi: pcl726: add support for the external interrupt signal

The ACL-6126 board supports an external interrupt signal on pin 17 of
its I/O connector (CN3). Add a new subdevice to this driver to support
asynchronous commands with this input.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>